The invention relates to a device for dewatering and drying suspensions. A dewatering and drying device of this type is known from EP 0591299.
In the known dewatering and drying device, the 0.3-3-mm moist solid particles sprayed radially at high speed at the discharge of the centrifuge, preferably a full-jacketed helical-conveyor centrifuge, are diverted by suitable means, for example, diverting surfaces or a suitable gas flow, in the axial direction of the centrifuge and guided by the gas flow on a helical flight path in the drying chamber. Here the sprayed solid particles are flowed around at a high relative speed by the drying gas and dried. The drying chamber is a concentric annular chamber. It is embodied by the outer drier housing, the inside, rotating drum jacket of the centrifuge, or an inside housing surrounding the drum and the two housing end walls. The outside walls of the concentric drying chamber are stationary, and must be sealed, at least at one location, against the rotating parts of the centrifuge inside.
The rotary seal between the centrifuge rotor and the surrounding drier housing must overcome and tolerate a high relative speed, a gas-difference pressure between the inside and outside, and displacement movements due to thermal expansions and vibrations. The seal is intended to prevent or minimize the escape of gases from the drier interior to the outside, or the entrance of secondary air from the outside to the inside.
It has been seen that the seal gap between stationary housing parts and rotating centrifuge parts changes in an unacceptable manner particularly because of thermal expansion during heating processes in the startup phase, or with the occurrence of vibrations or changes in the temperature of the drier housing. This can lead to contact between the seal surfaces from time to time, and damage to or destruction of the seal.
To avoid this, the gap width must be selected to be large enough that thermal expansions and displacements of the drier housing do not lead to touching of the contactless seals.
A further disadvantage is that the gap also changes due to vibrations of the dewatering centrifuge inside the drier, because the rotating and non-rotating parts of the seal are respectively secured to different seal carriers.
An excessively-large seal gap is particularly disadvantageous in the operation of the centrifuge drier with an inert-gas atmosphere, because the entrance of the secondary air noticeably increases the oxygen content of the inert drying gas.
A further disadvantage of the dewatering and drying device known from EP 0 591 299 relates to the diverting surfaces for the solid particles that are spun out of the rotating centrifuge. Despite the use of wall scrapers that are secured to the rotating centrifuge drum, deposits and encrustations can occur on the diverting surfaces, as well as in the drier housing or the downstream devices (washer, cyclone) if the centrifuge effects poor mechanical pre-dewatering of the suspension, or if the solid particles are very sticky and moist. In continuous drying operation, this causes disturbances and breakdowns, which is economically disadvantageous. Up to now, attempts have been made to effect positive changes in the moisture behavior and stickiness of difficult-to-dewater suspensions by mixing them with additives prior to centrifuging. This measure is, however, quite expensive.
It is the object of the invention to implement constructive measures to avoid disturbances in operation, as caused by either seal leakages between the drier housing and the centrifuge or deposits and encrustations of solid particles, in a dewatering and drying device of the type mentioned at the outset.
The invention provides the generation of a free dispersion of the pre-dewatered solids through mechanically-induced rolling turbulences of the drying gas; good distribution of the dispersed solid particles in the drying gas; the most uniform possible distribution of the particle concentration in the drying gas; and the blowing away of encrustation layers that may build up. The concentration of the small, dispersed, moist particles in the drier chamber should be uniform and low, and the relative speed of the hot gas in relation to the particles should be as high as possible to assure rapid drying of the moist solid particles in flight. For example, elements that induce the gas flow and assure a powerful turbulence in the vicinity of the surfaces in the drier chamber, which are at risk for encrustation, or at the diverting surfaces, are secured to the outside of the rotating centrifuge drum so as to project into the drier chamber. The surfaces of the work chamber walls in the drier can be polished or coated with an anti-adhesive to promote the prevention of encrustation. The directing and guiding sheets built into the drier chamber purposefully influence the flow of the hot gas to effect a uniform gas distribution, avoid dead spaces and assure an intensive contact of the hot gas with the moist solid particles. Perforated walls through which gas flows are also suitable for preventing encrustations due to moist, sticky solid particles if the hot gas flowing in keeps the sticky particles away from the walls until the particle surfaces have dried sufficiently and, having a lower moisture content, lose their tendency to stick. Particularly in organic clarification sludges having a pronounced adhesive phase, the tendency to stick is especially strong in certain moisture ranges and must be overcome in fractions of seconds in flight.
The invention further provides a sealing of the radial end walls of the drier housing against the rotating jacket surface of the centrifuge with a rotary seal, which can keep the seal gap very narrow without the risk of mechanical contact between the rotating and non-rotating work surfaces of the rotary seal, and thus damage to or destruction of these surfaces. A further advantage of the rotary seal is that even uncontrollable, large displacement and expansion movements of the drier housing during the heating or cooling phase of the centrifuge drier, or stronger vibrations during the operation, do not affect the sealing function, despite the narrow gap of the rotary seal. The escape of inside gases or solids or the entrance of secondary air into the inert drying gas is virtually entirely prevented by the narrow seal gap.
A further advantage of the invention is the avoidance of encrustations and baked-on buildup, even in difficult-to-dewater sludges. This expands the use and application range of the device of the invention to products which, after the mechanical dewatering, yield a solid that is extremely sticky or possesses a very high moisture content. Breakdowns caused by baked-on buildup as a result of excessively-moist mechanical pre-dewatering in the centrifuge, and the associated costs, are also avoided.